This study is concerned with the role of thyroxine on the biosynthesis of the liver enzymes and organelles involved in urea biosynthesis during differentiation and metamorphosis of the ammonotelic tadpole to the ureotelic frog. Identification of the primary molecular event(s) following thyroxine exposure is a major goal of this study. Related studies include: 1) mitochondrial and nuclear changes following thyroxine exposure; 2) mitochondrial heterogeneity and the specific functional roles of specialized mitochondria; 3) effects of thyroxine on mitochondriagenesis; 4) mechanism of action and comparative properties of ornithine transcarbamylase, the second enzyme involved in urea biosynthesis; and 5) factors involved in regulating the levels of the enzymes involved in urea biosynthesis.